Catheters are being used increasingly as a means for delivering diagnostic or therapeutic agents to internal target sites that can be accessed through the circulatory system. Often the site which one wishes to access by catheter is buried within a soft tissue, such as brain or liver, and is only reached by a tortuous route through small vessels or ducts--typically less than about 3 mm lumen diameter--in the tissue.
In one general method for accessing a deep-organ target site along a tortuous-path vessel, a torqueable guide wire and catheter are directed as a unit from a body access site to a tissue region containing a target site. The guide wire is bent at its distal end and may be guided by alternately rotating and advancing the wire along a tortuous, small-vessel pathway, to the target site. Typically the guide wire and catheter are advanced by alternately advancing the wire along a region of the pathway, then advancing the catheter axially over the advanced wire portion.
The difficulty in accessing such target body regions is that the catheter and guidewire must be quite flexible in order to follow the tortuous path into the tissue, and at the same time, stiff enough to allow the distal end of the catheter to be manipulated from an external access site, which may be as much as a meter or more from the tissue site.
Heretofore, catheter guidewires for use in guiding a catheter along a tortuous path have employed a variable-flexibility construction in which the distal end section of the wire is tapered along its length to allow greater flexibility at the wire's distal end region, where the sharpest wire turns are encountered. The tapered section of the wire is encased in a wire coil, such as a platinum coil, to increase the column strength of the tapered wire section without significant loss of flexibility in this region. Such guide wire constructions are disclosed, for example, in U.S. Pat. Nos. 3,789,841, 4,545,390, and 4,619,274.
The tapered guidewire construction just described is prepared, typically, by forming a fine-wire coil, cutting the coil to a desired length, and fastening the coil to the tapered distal end section of the guidewire, typically by soldering. This method of construction is relatively time consuming and costly in manufacture. Further, the solder attachment of the coil to the guidewire tip may crack during use, presenting the danger of having the coil separate from the wire within a vessel in the patient. Another limitation of the prior art construction is that the distal end section of the wire tends to kink on bending if the coil loses its relatively tight pitch, e.g., by being irreversibly stretched during use.